primary key changes value after update - becomes 0 - php

Question

Well it seems I've found the problem.
Because the values for the fields where automatically produced by taking substrings of the names of an HTML form fields, one of the values was appearing as id (it can be seen in the echo I've posted bellow). The update as it seems, didn't fail but produced this unexpected behavior.
Thanks to all who tried to help and sorry for the stupid question.
After I execute an update statement like
UPDATE tablename SET field='value' WHERE field='value'
without ever touching my primary key, it change's value from what it was before to 0.
Any ideas?
MySQL Server version: 5.5.37-0+wheezy1
This is the code that generates the query
$query2 = "UPDATE student SET ";
foreach ($_POST as $the_key => $a_post_arg) {
if (strcmp($the_key, "student_password") === 0) {
//without activation
$a_post_arg = md5($a_post_arg);
//with activation
//$a_post_arg = "Not activated!";
}
if (strcmp($the_key, "student_registration_year") === 0)
$a_post_arg = $a_post_arg . "-00-00";
if (strcmp(substr($the_key, 0, 14), "student_stats_") === 0 || strcmp($the_key, "student_validationImageTextfield") === 0) {
$student_stats[$the_key] = $a_post_arg;
continue;
}
if (strcmp(substr($the_key, 0, 7), "student") === 0 && strcmp($the_key, "student_email_retype") !== 0 && strcmp($the_key, "student_password_retype") !== 0) {
if (strcmp($the_key, "student_email") !== 0) {
if (strcmp($the_key, "student_select_dept") === 0) {
$query2 .= "dept='" . addslashes($a_post_arg) . "', ";
} else if (strcmp($the_key, "student_semester") === 0) {
$query2 .= "studying_semester='" . addslashes($a_post_arg) . "', ";
} else if (strcmp($the_key, "student_father_name") === 0) {
$query2 .= "fathers_name='" . addslashes($a_post_arg) . "', ";
} else if (strcmp($the_key, "student_academic_id") === 0) {
$query2 .= "academicIDNumber='" . addslashes($a_post_arg) . "', ";
} else {
$query2 .= substr($the_key, 8) . "='" . addslashes($a_post_arg) . "', ";
}
}
}
}
$query2 .= "status='registered' WHERE email='" . $_POST['student_email'] . "';";
This is an echo of $query2. The PK for the table is the auto increment field id and the email field.
query 2 = UPDATE student SET name='Όνομα', surname='Επώνυμο', dob='1970-09-09',
fathers_name='Ονοματεπώνυμο πατέρα', mother_name='Ονοματεπώνυμο μητέρας',
nationality='Υπηκοότητα', adt='Α.Δ.Τ.', password='0cc175b9c0f1b6a831c399e269772661',
dept='biology', id='Αριθμός μητρώου', studying_semester='6', registration_year='2001-00-00',
atlas_id='1234', academicIDNumber='123456789012', perm_address_road='Οδός',
perm_address_number='Αριθμός', perm_address_area='Περιοχή/Πόλη', perm_address_PObox='Τ.Κ.',
perm_address_Country='Χώρα', study_address_road='Οδός', study_address_number='Αριθμός',
study_address_area='Περιοχή/Πόλη', study_address_PObox='Τ.Κ.', study_address_Country='Χώρα',
telephone='[phone]', cellphone='[phone]', fax='[phone]', afm='Α.Φ.Μ.',
eforia='Δ.Ο.Υ.', amka='Α.Μ.Κ.Α.', amika='Α.Μ.ΙΚΑ', status='registered' WHERE email='20#send.com';